Interconnect structure for high frequency signal transmissions

ABSTRACT

A higher aspect ratio for upper level metal interconnects is described for use in higher frequency circuits. Because the skin effect reduces the effective cross-sectional area of conductors at higher frequencies, various approaches are described to reduce the effective RC delay in interconnects.

RELATED APPLICATION

This application claims priority to U.S. Provisional Application Ser.No. 61/036,934, filed Mar. 15, 2008, whose contents are expresslyincorporated herein by reference.

TECHNICAL FIELD

Aspects of the invention relate to providing an interconnect structurefor high frequency semiconductor devices.

BACKGROUND

Semiconductor devices are used in a variety of fields includingcommunication fields. By using different types of semiconductors(including, but not limited to, indium phosphide and gallium arsenide),devices have been created to for higher frequency applications.

Part of a device is the device's interconnect structure. Conventionaldevices use larger conductors as the current in a signal increases or asthe distance a signal needs to be conveyed increases. Larger conductorscan consume significant real estate. Also, because of phenomena(including the skin effect) that occur as frequencies increase, largerconductors alone cannot provide interconnect structures in integratedcircuits that also further device miniaturization.

SUMMARY

Aspects of the invention pertain to interconnect structures for use withhigher frequency semiconductor devices. In one example, aspect ratiosfor upper level interconnect lines are increased in comparison to lowerlevel interconnect lines. In another example, multiple line segments areconnected by connecting plugs to overcome maximum pitch limits. Theseand other examples are described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example of conductors with different cross-sectionalareas and the same aspect ratio.

FIG. 2 shows a relationship between frequency and skin depth.

FIG. 3 shows an example of conductors with different cross-sectionalareas and the same aspect ratio in accordance with one or moreembodiments.

FIGS. 4A, 4B, 4C, and 4D show various interconnects in accordance withone or more embodiments.

FIG. 5 shows another interconnect structure in accordance with anotherembodiment.

FIG. 6 shows examples of how multiple conductive plugs may be used toconnect interconnects in accordance with one or more aspects of theinvention.

DETAILED DESCRIPTION

One or more aspects of the invention relate to interconnects for usewith high frequency semiconductor devices.

It is noted that various connections are set forth between elements inthe following description. It is noted that these connections in generaland, unless specified otherwise, may be direct or indirect and that thisspecification is not intended to be limiting in this respect.

Conventional interconnects are shown in FIG. 1 in an elevation viewwhere lower level interconnects have a small cross-sectional area,midlevel interconnects have larger cross-sectional areas, and top levelinterconnects have the largest cross-sectional areas. In FIG. 1,interconnects 101-108 are shown as having an aspect ratio of h/w (asshown in FIG. 1, about 1.5:1). Aspect ratios between 1:0 and 2:0(generally, 1.7:1 to 2.0:1) are common. For reference, the height isdesignated as “h” and the width is designated as “w.” For interconnects101-108, each is represented as having a cross-sectional area of h*w (orhw). Interconnects 101-108 are separated by a distance “d.” For purposesof explanation and comparison, distance d is set to be the same as widthw.

Each interconnect 101-108 has an associated resistance and capacitance.Resistance can be calculated by the following equation (1):

$\begin{matrix}{R = \frac{\ell \cdot \rho}{A}} & (1)\end{matrix}$

where:

-   -   l is the length of the conductor, measured in meters;    -   A is the cross-sectional area, measured in square meters; and    -   ρ (Greek: rho) is the electrical resistivity (also called        specific electrical resistance) of the material, measured in        Ohm·meter. Resistivity is a measure of the material's ability to        oppose electric current.

With respect to interconnects 101-108, the resistance is then:

$\begin{matrix}{R = {\frac{\ell \cdot \rho}{A} = \frac{\ell \cdot \rho}{h \cdot w}}} & (2)\end{matrix}$

In short, the resistance R for a given interconnect decreases as thecross-sectional area of the interconnect increases.

Capacitance between the interconnects 101-108 can be determined if thegeometry of the interconnects 101-108 and the dielectric properties ofthe insulator between interconnects 101-108 is known. For instance,assuming interconnects 101-108 are parallel plates, with a height h anda length l separated by a distance d is approximately equal to thefollowing equation:

$\begin{matrix}{C = {ɛ_{r}ɛ_{0}\frac{A}{d}}} & (3)\end{matrix}$

where

-   -   C is the capacitance in farads F;    -   A is the area of overlap of the two plates measured in square        meters (height h being shown in FIG. 1 and length l into the        page);    -   ∈_(r) is the relative static permittivity (sometimes called the        dielectric constant) of the material between the plates,        (vacuum=1);    -   ∈₀ is the permittivity of free space where ∈₀=8.854×10⁻¹² F/m;        and    -   d is the separation between the plates, measured in meters.

Equation (3) is a good approximation if d is small compared to the otherdimensions of the plates so the field in the capacitor over most of itsarea is uniform, and the fringing field around the periphery provides asmall contribution.

In this regard, the capacitance between interconnects 101-108 can beapproximated as follows:

$\begin{matrix}{C = {{ɛ_{r}ɛ_{0}\frac{A}{d}} = {ɛ_{r}ɛ_{0}\frac{h \cdot \ell}{d}}}} & (4)\end{matrix}$

In short, the capacitance C between two interconnects of 101-108increases as the cross-sectional area between the interconnectsincreases and decreases as the distance between the interconnectsincreases. Assuming length l is constant, then the capacitance increaseswith the height of each interconnect.

As the frequency of a signal increases, the signal is increasinglysusceptible to RC delays. An RC delay (the value of time constant τ is ameasure of the time needed to charge a capacitor through a resistor:τ=R·C  (5)

The RC delay can then be expressed as generally proportional to squareof the length and inversely proportional to the width and distancebetween the interconnects:

$\begin{matrix}{{\tau \approx {\frac{\ell}{h \cdot w} \cdot \frac{h \cdot \ell}{d}}} = \frac{\ell^{2}}{w \cdot d}} & (6)\end{matrix}$

The typical digital propagation delay of a resistive wire is about halfof R times C; since both R and C are proportional to wire length, thedelay scales as the square of wire length.

Turning now to interconnects 109-112 and interconnects 113-114, thecross-sectional area of each of interconnects 109-112 is 2h*2w=4hw,while the aspect ratio is 2h/2w=h/w, namely, the same for interconnects101-108. The cross-sectional area of each of interconnects 113-114 is4h*4w=16hw, while the aspect ratio is 4h/4w=h/w, namely, the same forinterconnects 101-108 and interconnects 109-112.

The RC delay for interconnects 109-112 can be approximated to be ¼ ofthe RC delay of the interconnects 101-108 as τ≈1/(2w2d)=¼wd. The RCdelay for interconnects 113-114 can be approximated to be 1/16 of the RCdelay of interconnects 101-108 as τ≈1/(4w4d)= 1/16wd.

From the above calculation of the RC delay, it would appear thatincreasing the size and spacing of interconnects is enough to limit RCdelays. However, this approach fails as the frequency of a signalincreases. As the frequency of a signal increases, the skin effectbegins to appear. The skin effect is the tendency of an alternatingcurrent signal to distribute itself within a conductor so that thecurrent density near the surface is greater than at its core. Theeffective resistance increases with the frequency of the current. Theskin effect is small at low frequencies (for instance, below 5 GHz) forinterconnects on integrated circuits. However, as the signal increases,the skin effect also increases. Aspects of the invention may be appliedto semiconductor devices (and their associated circuitry) that work withhigh frequency signals equal to and greater than 5 GHz or withsemiconductor devices in which interconnects are susceptible to the skineffect as frequencies increase. The ramifications of the skin effectbecome more pronounced as the length of the interconnect increases (duein part to the increase in effective capacitance between interconnectsdue to the increased surface area between interconnects). In thisregard, longer interconnects (for instance, those at a higher level inthe semiconductor device) that carry high frequency signals may benefitfrom one or more aspects of the invention.

For instance, at microwave frequencies, most of the current in a goodconductor flows in a thin region near the surface of the conductor. Forcomparison, a 10 GHz microwave frequency is approximately four timeshigher than the frequency used common devices including Bluetooth,wireless access points, microwave ovens, and satellite television (whichoperate around the 2.4 GHz band). These devices have approximately twotimes as much penetration into the surface of a conductor compared tothe penetration of signals at frequencies around 10 GHz.

FIG. 2 shows a general calculation of the skin depth as frequencyincreases for conductors. The frequency range from 1 GHz to 64 GHz isshown on the x-axis and the depth from the surface of a conductor isshown on the y-axis in the range from 0.0 to 1.6 μm. FIG. 2 shows threecurves, at 90%, 70%, and 50% of the current density into the conductoras frequency increases. Notably, for example, the 50% current densityvalue drops from approximately 1.4 at 1 GHz to 0.5 at 8 GHz. The effectis that larger conductors as shown in FIG. 1 have less of an effect athigher frequencies of reducing resistance than at lower frequencies. Inthis regard, RC delays at higher frequencies are more significant thanat lower frequencies.

High speed circuits benefit from interconnects with low resistivecapacitive (RC) delays. Conventional interconnect structures as shown inFIG. 1 keep aspect ratios (height/width) of minimum width interconnectsalmost constant regardless of the interconnection level. The result isthat the larger interconnects (109-112 and 113-114) are less efficientat conveying a high frequency signal than their larger size wouldsuggest.

FIG. 3 shows various interconnects in an elevation view in accordancewith one or more aspects of the invention. FIG. 3 includes interconnects301-308 with a height h, a width w, and separated by distance d fromeach other. The cross-sectional area of each is hw and the aspect ratiois h/w. Interconnects 301-308 are similar to interconnects 101-108 ofFIG. 1. It is appreciated that insulators (for example, silicon dioxideand other known insulators commonly positioned between interconnects)are positioned between adjacent interconnects.

FIG. 3 also shows interconnects 309-312 with a height of 4h and a widthof w. For purpose of explanation, the distance between interconnects309-312 is shown to be three times the distance d (namely, 3d). Thecross-sectional area of each of interconnects 309-312 is 4hw and theiraspect ratio is 4h/w. Here, the cross-sectional area (at 4hw) is thesame as the cross-sectional area of interconnects 109-112. However, theaspect ratio is twice that of interconnects 109-112. In that skin depthdecreases as frequencies increase, the effective resistance ofinterconnects 309-312 can be lower based on a greater surface area ofinterconnects 309-312. Because of the cross-sectional volume ofinterconnects 309-312 is added to the height, the effective distancebetween interconnects 309-312 is increased. In the example of FIG. 3,the distance is increased from 2d to 3d. The effect of increasing thedistance between the interconnects while keeping the width to w (insteadof increasing the width to 2w as shown in FIG. 1) changes the RC delay.It is believed that the RC delay would be less for the tallerinterconnects as the separation distance between them increase.Similarly, the RC delay is expected to be less for interconnects 313-314than for interconnects 113-114 because of the increase in distancebetween the interconnects despite their increase in height. Thedifference is that the skin effect (by limiting the effectivecross-sectional area of the interconnect at high frequencies) increasesthe effective resistance of the interconnect. By increasing the distancebetween the interconnects, the RC delay caused by the proximity of theinterconnects is reduced.

In FIG. 3, the aspect ratio of the interconnects increases at highermetallization layers. For instance, standard chips have four or moremetal layers. In one aspect of the invention, the increase in aspectratio only occurs in the upper metal layers. Because of the extramaterials required to make the higher aspect ratio interconnects,increase in height, the increased aspect ratio for interconnects isreserved in this instance to the upper level or upper levels ofinterconnects.

Another aspect of the invention shown in FIG. 3 is that the conventionalapproach of separating interconnects by a distance equal to their widthis not followed. As shown in FIG. 3, the upper layer interconnects areseparated by distances larger than their widths. For instance,interconnects 309-312 are separated by distances three times theirwidth. Interconnects 313 and 314 are separated by distances seven timestheir width. In examples of FIG. 3, the increase in separation distancewas due to attempting to not modify the general placement of the upperlevel interconnects. It is appreciated that other distances betweeninterconnects may be used based on considerations of the aspect ratio,the dielectric material between the interconnects, the height (oreffective height or surface area) of the interconnects, and thefrequency or frequencies of the signals passing through theinterconnects. For instance in addition to the above distances, theseparation distance may be two times the width of the interconnect, fourtimes the width of the interconnect, five times the width of theinterconnect, six times the width of the interconnect, eight times thewidth of the interconnect, nine times the width of the interconnect, andthe like.

Yet another aspect of the invention shown in FIG. 3 is the pitch used tocreate the higher aspect ratio for the upper level interconnects islarger than the minimum pitch of other lower level interconnects.

In one embodiment of the invention, the increase in aspect ratios maystart in all the metal layers. In an alternative embodiment of theinvention, the increase in aspect ratios may start at the fifth orhigher metal layer.

One benefit by using the interconnect structure herein is that thedevice can be fabricated by conventional copper interconnect process anddevice technology.

In another aspect of the invention, the aspect ratio changes between thelower interconnect lines and the upper interconnect lines. In contrast,conventional metal interconnect structure of high performance logic LSIhave larger minimum dimensions (wire width and height) in upper levelwith almost consistent interconnect aspect ratio (height/width) at thelowest level. This is because upper level interconnects are used forlong distance signal propagation and need to be reduced RC delay. On theother hand and as described above, high frequency signals propagate onlynearby surface of interconnect. Even though upper level interconnectshave larger minimum dimensions with consistent aspect ratio and thelarger intersectional area, the benefit of resistance reduction issmaller than increase of intersectional area. This tendency becomes moresignificant in larger dimension interconnects and higher frequencysignals situations.

As used in one or more aspects of the present disclosure, higher aspectratio in upper interconnects help to minimize impact of the skin effect.Here, the coupling capacitance increase can be prevented even if tallerinterconnects provide larger facing areas to adjacent interconnects byreducing the interconnect width and increasing the distance to adjacentinterconnects.

FIGS. 4A-4D show various interconnects in an elevation view. In FIG. 4A,a single interconnect 401 is shown. The aspect ratio of an increasedaspect ratio interconnect may be between 3.0:1 and the upper limit ofwhat a formation process may support. For instance, conventionalsemiconductor formation processes can support an aspect ratio of 10:1(albeit with difficulty). The conventional processes include forming ametal layer then etching away unwanted portions, resulting in highaspect ratio interconnects. Also, conventional processes include forminga trench and filling it with a metal. In this latter example, fillingall trenches in a single operation can be difficult due in part toincomplete filling of a portion of a trench. Better results may occurthrough filling a single trench at a time. However, separately exposing,filling, etching, then masking each trench can be time-consuming andcostly.

To accommodate greater aspect ratios (for instance, aspect ratiosgreater than 3:1), FIGS. 4B and 4C show interconnects connected by plugsto form interconnects with higher aspect ratios. FIG. 4B shows a firstinterconnect 402, a conductive plug 403, and another interconnect 404electrically connected to the first interconnect 402 via the conductiveplug 403. FIG. 4C shows a first interconnect 405, a first conductiveplug 406, a second interconnect 407, a second conductive plug 408, and athird interconnect 409. The result is high aspect ratio interconnect.

FIG. 4D shows an interconnect made of a number of smaller interconnectsand conductive plugs. FIG. 4D shows a first interconnect 410, a firstconductive plug 411, a low aspect ratio interconnect 412, electricallyconnected to interconnect 410 by conductive plug 411, conductive plugs414 and 415 on opposite ends of the top surface of interconnect 412,interconnect 416 connected to low aspect ratio interconnect 412 viaconductive plug 414, and interconnect 417 connected to low aspect ratiointerconnect 412 via conductive plug 415.

An advantage of the composite interconnect of FIG. 4D is its largesurface area compared to its height. Further additional surface areaadvantages may be achieved through additional connections of smallersets of interconnects in various geometric shapes (for instance, the “Y”shape of FIG. 4D, a “T” shape, stacked horizontal plates, and the like).

FIG. 5 shows yet another technique of increasing the surface area of aninterconnect. Here, a lower interconnect 501 is connected to conductiveplug 502. A “U” shaped interconnect 503 is connected to interconnect 501through conductive plug 502. Interconnect 503 is then connected toanother interconnect 505 through conductive plugs 504 and 506. It isappreciated that only one of conductive plugs 504 and 506 may be used.However in this example, both conductive plugs are used to permit moreof the surface area of interconnect 503 to be used because of the skineffect as compared to a sole conductive plug (504, for example). Aninsulator 507 may partially (or completely) fill the inside ofinterconnect 503.

FIG. 6 shows examples of how multiple conductive plugs may be used toconnect interconnects in accordance with one or more aspects of theinvention.

FIG. 6 shows a top-down view of an interconnect structure. A firstinterconnect 601 is arranged in a first direction, running from the topto the bottom of the Figure. An insulator 602 is shown on top ofinterconnect 601. Insulator 602 may silicon dioxide or any otherinsulator as known in the art. Interconnect 603 is arranged on top ofinsulator 602 from the left to right of the Figure. Conductive plugs 604are shown connecting interconnects 601 and 603. Here multiple conductiveplugs are used to decrease any resistance caused by the conductive plugs604 in an electrical path from interconnect 601 to interconnect 603.

In another example, the upper-level interconnect 603 is replaced byinterconnect 605. Interconnect 605 is arranged parallel to interconnect601. Because of the longer parallel length between interconnect 601 andinterconnect 605, additional conductive plugs 604 may be used to furtherconnect interconnect 601 and interconnect 605. This will help furtherreduce any resistance between interconnect 601 and interconnect 605.

This specification describes the interconnects as metal lines (includingfor instance copper). In an alternative embodiment, the interconnectsmay be metallized lines and not purely metal.

Although the subject matter has been described in language specific tostructural features and/or methodological acts, it is to be understoodthat the subject matter defined in the appended claims is notnecessarily limited to the specific features or acts described above.Rather, the specific features and acts described above are disclosed asexample forms of implementing the claims. Numerous other embodiments,modifications, and variations within the scope and spirit of theappended claims will occur to persons of ordinary skill in the art froma review of this disclosure.

1. A semiconductor device comprising: a first interconnect at a firstlevel with a first aspect ratio; a second interconnect at a second levelwith a second aspect ratio; where the second level is higher in thesemiconductor device than the first level; one or more conductive plugsconfigured to electrically connect the first interconnect and the secondinterconnect, wherein the second aspect ratio is greater than 3.0:1 andthe first aspect ratio is less than the second aspect ratio.
 2. Thesemiconductor device according to claim 1, wherein the first aspectratio is less than 2.0:1.
 3. The semiconductor device according to claim1, wherein the second level and the first level are separated by atleast three additional levels of interconnects.
 4. The semiconductordevice according to claim 1, wherein the semiconductor device is a highfrequency semiconductor device configured such that interconnects areadapted to convey signals equal to and greater than 5 GHz.
 5. Thesemiconductor device according to claim 1, wherein the secondinterconnect includes at least two interconnects connected by conductiveplugs.
 6. The semiconductor device according to claim 1, wherein thesecond interconnect includes a concave portion at least partially filledwith an insulator.
 7. The semiconductor device according to claim 1,wherein the second interconnection layer is a metal line.
 8. Thesemiconductor device according to claim 1, wherein the secondinterconnection layer is used for long distance signal propagation.